GUIDE WAVE ANALYSIS AND FORECASTING - WMO

necessary to forecast waves for a particular location and
also to perform a spatial wave analysis (i.e. analyse a
wave chart) by manual methods. A great deal of experience
is necessary to analyse an entire chart within
reasonable time limits, mainly because one has to work
with constantly changing wind conditions.
Generally, one starts from known wave and wind
conditions, say 12 h earlier, and then computes, using
the present analysed wind chart, the corresponding
wave chart. In cases of sudden wind changes, the intermediate
wind chart from 6 h earlier may also be
needed. The forecast wind in the generation area and
the forecast movement of the generation area are also
necessary to produce the best wave forecast over the
next 24 to 36 h.
4.2 Some empirical working procedures
Three empirical procedures are briefly discussed in this
section. They are concerned with freshening of the wind
at constant direction, changing wind direction, and
slackening of the wind. These procedures are useful
when time is short for preparing a forecast.
4.2.1 Freshening of the wind at constant
direction
Freshening wind at constant direction is a frequent
occurrence and the procedure described in Section 4.3.4
should be used. For quick calculations: subtract onequarter
of the amount the wind has increased from the
new wind speed, and work with the value thus obtained.
Example: the wind speed has increased from 10 to 20 kn
(1.94 kn = 1 m/s) over the last 12 h; to compute the
characteristic wave height, use a wind speed of 17.5 kn
over a duration of 12 h. When sharp increases of wind
speed occur, it is advisable to perform the calculation in
two stages.
4.2.2 Changing wind direction
If the direction changes 30° or less, wave heights and
periods are computed as if no change had occurred; the
wave direction is assumed to be aligned with the new
direction. At greater changes, the existing waves are
treated as swell, and the newly generated waves are
computed with the new wind direction.
4.2.3 Slackening of the wind
When the wind speed drops below the value needed to
maintain the height of existing waves, the waves turn into
TABLE 4.2
Characteristic parameters of wind waves
–
Hc Tc Tp TH1/3 f
Range of
p
Hmax
periods
(m) (s) (s) (s) (s –1 ) (s) (m)
5 9 10.3 9.3 0.097 5 –15 10
WAVE FORECASTING BY MANUAL METHODS 45
swell and should be treated as such. As a first approximation,
swell may be reduced in height by 25 per cent per
12 h in the direction of propagation. For instance, swell
waves 4 m high will decrease to 3 m in 12 h.
4.3 Computation of wind waves
4.3.1 Determining sea-state characteristics for a
given wind speed and fetch
Problem:
Determine the characteristics of the sea state for a wind
speed of 15 m/s (about 30 kn), with a fetch of 600 km
(about 325 nautical miles (n.mi.)) and after a duration of
36 h.
Solution:
According to the diagram given in Figure 4.1, the fetch
is the limiting factor. For a fetch of 600 km, the characteristic
wave height (Hc) is 5 m and the characteristic
period (Tc) is 9 s.
Other characteristics can be obtained as well. From
the JONSWAP spectrum (see Section 1.3.9, Figure 1.17,
and Equation 4.1), we can find the peak frequency and
period, and from them the range of important wave
periods and the period of the highest one-third of the
waves (significant wave period, T – H1/3 ). From T– H1/3 we
can derive the highest wave in a 6-h period.
The peak frequency fp is given by:
–0.6 0.2 fp = 0.148 Hm0 u (4.1)
where Hm0 is the model wave height in metres, and u is
wind speed in metres per second. The model period is
Tp =1/fp. Assume Hm0 ≈ Hc = 5 m, and u = 15 m/s, then
fp = 0.097 s –1 , and Tp = 10.3 s.
To determine T – H1/3 , we use Goda’s (1978) results in
an approximate form:
T – H1/3 ≈ 0.9 Tp . (4.2)
For our problem, T – H1/3 ≈ 0.9 x 10.3 = 9.3 s.
The range of important wave periods can be determined
from Figure 1.17, where f (= 1/T) varies from
about 0.7 fp to about 2.0 fp. This translates to a range of
5 to 15 s. The maximum energy in the spectrum will be
near the period of 10 s.
For a record of wave heights with about 2 000 wave
measurements, we can use the following approximation
to find the maximum wave height:
Hmax ≈ 2.0 H – 1/3 ≈ 10 m.
Table 4.2 presents all these results.
4.3.2 Determining sea state for an increasing
wind speed
Problem:
An aeroplane has had to ditch at sea 200 km from shore.
The closest ship is positioned 600 km from shore. The
wind speed over the last 24 h has been steady at 17 m/s.